Investigate the ability of second-trimester placental volume measured sonographically to predict birth size. A total of 712 women were recruited from the antenatal clinic of the University Hospital of the West Indies; 561 fulfilled the study criteria and progressed to delivery. Placental volume and fetal anthropometry (biparietal diameter, head and abdominal circumferences, and femoral length) were measured sonographically at 14, 17, and 20 weeks. The main outcome measures were infant birth and placental weights, length, head, chest, and abdominal circumferences at birth. Placental volume in the second trimester was positively associated with all birth measurements. Of the fetal measurements at 14 and at 17 weeks, head circumference was the strongest predictor of birth weight (B [slope of the regression line] =.095, P =.014 at 14 weeks; B =.118, P =.012 at 17 weeks), but at 20 weeks, abdominal circumference was the strongest. However, at each age, placental volume was the strongest determinant of birth weight, and improved the prediction based only on fetal measurements. The odds ratio for low birth weight (under 2500 g) increased by 1.68 (95% confidence interval 1.05, 2.69, P = 0.03) for every standard deviation decrease in placental volume at 14 weeks' gestation. The present study suggests that low birth weight was often preceded by small placental volume in the second trimester. Placental volume may be a more reliable predictor of size at birth than fetal measurements, and may be useful in early identification of the fetus at risk in the perinatal period.....read more